In a typical cellular radio communications system (wireless communication system), an area is divided geographically into a number of cell sites, each defined by a radio frequency (RF) radiation pattern from a respective base transceiver station (BTS) antenna. The base station antennae in the cells are in turn coupled to a base station controller (BSC), which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (MSC) or packet data serving node (PDSN) for instance. The switch or gateway may then be coupled with a transport network, such as the public switched telephone network (PSTN) or a packet-switched network (e.g., the Internet).
When a mobile station (such as a cellular telephone, pager, or appropriately equipped portable computer, for instance) is positioned in a cell, the mobile station communicates via an RF air interface with the BTS antenna of the cell. Consequently, a communication path is established between the mobile station and the transport network, via the air interface, the BTS, the BSC and the switch or gateway.
With the explosive growth in demand for wireless communications, the level of call traffic in most cell sites has increased drastically over the years. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of sectors, each defined respectively by radiation patterns from directional antenna components of the respective BTS, or by respective BTS antennae. These sectors (which can be visualized ideally as pie pieces) can be referred to as “physical sectors,” since they are physical areas of a cell site. Therefore, at any given instance, a mobile station in a wireless network will typically be positioned in a given physical sector and will be able to communicate with the transport network via the BTS serving that physical sector.
As a mobile station moves between sectors or other coverage areas of a wireless communication system, or when network conditions change or for other reasons, the mobile station may “hand off” from operating in one coverage area to operating in another coverage area. In a usual case, this handoff process is triggered by the mobile station monitoring the signal strength of signals that it is receiving in various available coverage areas, and the mobile station or the BSC determining when one or more threshold signal strength criteria are met. For instance, the mobile station may monitor signal strength in various available coverage areas and notify the BSC when a given coverage area has a signal strength that is sufficiently higher than the coverage area in which the mobile station is currently operating. The BSC may then direct the mobile station to hand off to that other coverage area.
In practice, communications over the air interface between a BTS (base station) and a mobile station will comply with a defined air interface protocol or “access technology.” Numerous such protocols are well known in the art, and others will be developed in the future. Examples of existing protocols include CDMA (e.g., 1xRTT, 1xEV-DO), iDEN, TDMA, AMPS, GSM, GPRS, UMTS, EDGE, WiMAX (e.g., IEEE 802.16), LTE, microwave, satellite, MMDS, Wi-Fi (e.g., IEEE 802.11), and Bluetooth. Each protocol may define its own procedures for handoff between coverage areas and may define other procedures and parameters related to air interface communication.
Furthermore, in some cases, more than one air interface protocol might be implemented in a given market area. For example, a given market area might provide both legacy CDMA 1xRTT coverage under a standard protocol such as EIA/TIA/IS-2000 Rel. 0, Rel. A or another version thereof (hereafter “IS-2000”) and also high data rate 1xEV-DO coverage according to a standard protocol such EIA/TIA/IS-856 Rel. 0, Rel. A, or another version thereof (hereafter “IS-856”). In a system that provides two or more air interface protocols in a single area, a mobile station might not only hand off between coverage areas under a common air interface protocol (e.g., between IS-2000 sectors) but may also hand off between the different air interface protocols (e.g., between IS-2000 and IS-856) within a given physical location or when moving between locations. Handoff between different air interface protocols is known as “vertical” handoff.